A conventional analog-to-digital converter (ADC) usually converts an analog input signal VIN to a digital output signal DOUT based on a bandgap reference voltage VREF. The digital output signal DOUT indicates a ratio of the analog input signal VIN to the bandgap reference voltage VREF. The bandgap reference voltage VREF can be generated by a bandgap reference circuit with first-order, second-order, or higher-order temperature compensation. Disadvantageously, if the bandgap reference voltage VREF is generated by a bandgap reference circuit with low-order temperature compensation, the bandgap reference voltage VREF may vary over a predefined temperature range. For example, a bandgap reference voltage VREF generated by a bandgap reference circuit with first-order temperature compensation varies by a few millivolts over the range of −55° C. to 125° C. Such variation can cause an output error to the ADC. If the bandgap reference voltage VREF is generated by a bandgap reference circuit with high-order temperature compensation, the bandgap reference voltage VREF may be substantially constant over the predefined temperature range. However, the structure of a bandgap reference circuit with high-order temperature compensation is complicated. If the bandgap reference circuit is integrated with the ADC into a single chip, design and testing of the bandgap reference circuit are sensitive to the process or change in the process performed by the ADC, which will increase the cost of automatic test equipment (ATE) and increase the time for testing performed by the ATE.